From the ALSA wiki
Make sure you disable Oss in your Kernel configuration. If you don't, your system may try to use these drivers rather than the ALSA ones. If you see a message about...
Sound card not detected
and you are sure you have the correct ALSA driver, this is the reason. You have to go to
/etc/modules.conf to disable (comment with a #) the lines that correspond to the plain Oss sound driver kernel modules. Check to see if you still have them by doing an...
If you do, remove them with...
If you don't unload them the alsa driver will not be able to start, because the Oss driver will be using the hardware resources associated with your SoundCard.
Note: Some kernels may be compiled with the OSS emulation modules compiled directly into the kernel. In this case you will need to recompile the kernel and change them so they are compiled as modules rather than in the kernel itself. More information about Kernel compiling can be found at http://www.tldp.org/HOWTO/Kernel-HOWTO/ and http://www.digitalhermit.com/linux/Kernel-Build-HOWTO.html